Wireless communication systems are typically designed to operate in harsh environments under stringent constraints. The harsh environments include harsh physical environments and operating conditions as well as harsh electrical environments.
Wireless communication systems are typically constrained to operating in specified frequency bands, which may be licensed or unlicensed frequency bands. Wireless receivers need to have the ability to support communication links in the presence of in-band noise and interference sources as well as out of band interference and noise sources. Similarly, wireless transmitters need to have the ability to support communication links while operating within emissions limits. The emission limits defined in an operating specification or standard may constrain in-band emissions as well as out of band emissions.
A wireless transmitter may be limited to a maximum radiated power in the operating band. Where the transmit band is frequency division multiplexed, the transmitter contribution to adjacent channel interference is typically constrained. The transmitter out of band emissions are also typically constrained, such that the transmitter does not contribute significant interference to frequency bands that may be dedicated to different communication systems operating under different standards.
The wireless communication system, and in particular the transmitters in a wireless communication system, typically are designed to comply with out of band emission constraints under worst case operating conditions. Typically, a transmitter generates the highest level of out of band emissions when operating at a maximum transmit power level.
Under worst case conditions, a transmitter may be required to have over 100 db of difference between the transmit power level and the permissible out of band power emissions level. The transmitter's ability to meet an out of band emissions constraint is further complicated by the proximity of the operating frequency with the band edges. The ability to successfully limit out of band emissions is degraded as the transmitter operating frequency nears the band edges.
Various brute force techniques are known for limiting out of band emissions. However the application of the various techniques in an attempt to satisfy an emissions constraint may substantially degrade transmitter performance to a point that makes the brute force solution impractical.
For example, series filtering can be added to a transmitter to increase the rejection of out of band emissions. However, because filters contribute some level of passband attenuation, the requisite level of out of band rejection may substantially limit the transmit power capabilities of the transmitter. Similarly, the proximity of a transmit operating band to an emissions limited band may make implementation of a high order filter impractical.
Other techniques for reducing out of band emissions in a specified band, such as transmitting over a different operating frequency or implementing a different modulation technique, may be impractical for use in licensed or other wise regulated bands, where the transmit band and operating parameters are standardized or otherwise specified.
It is desirable to reduce out of band emission while maintaining the ability to satisfy the various requirements set forth in an operating standard or specification.